China National Offshore Oil Corp. said Monday that it's making nine offshore blocks in the South China Sea available for joint operation with foreign companies.

The blocks, in water 300-4,000 meters deep, cover an area of 160,124.38 square kilometers, the company said in a statement on its website.

Seven blocks are located in the Zhongjianan Basin and two in areas covering parts of the Wan'an and Nanweixi basins, the company said.

The move comes as Cnooc has been striving to reduce its reliance on foreign expertise by recently developing its own deepwater capabilities as part of efforts to locate more domestic oil and gas reserves in more challenging offshore locations. With the official launch of its first ultra-deepwater drilling platform May 9, Cnooc for the first time independently drilled deepwater wells without the help of a foreign partner or equipment.

The company offered 19 offshore blocks for joint operation with foreign companies in 2011 and 13 offshore blocks in 2010.

Cnooc, China's largest offshore oil and gas producer by capacity, has been collaborating with international companies to develop its offshore oil fields since the 1980s due to technical difficulties and economic risks involved in offshore oil development, according to China Daily. Cnooc owns 51% of the oil and gas output from the fields, while the foreign companies own the remainder as a result of providing capital and technology during the exploration stage.

Cnooc has traditionally relied on foreign partners such as Canada's Husky Energy Inc. (HSE.T), U.S.-based Chevron Corp. (CVX) and U.K.-based BG Group (BG.LN) to explore and operate deepwater blocks off China.

Although Husky achieved considerable success finding deepwater gas in the Liwan 3-1 field in 2006, the lack of seismic data, high cost of drilling exploratory wells and less-than-lucrative contract terms have discouraged foreign companies from partnering to develop China's deepwater blocks.

Last year, Cnooc publicly offered its first deepwater oil blocks to foreign companies for joint operation, but only Italian oil major Eni SpA (E) made a successful bid, announcing in April that year that it had signed a production-sharing contract with Cnooc for a block located 400 kilometers off Hong Kong.

A third of China's oil and gas reserves are located in the South China Sea, and 70% of those are located in a 1.537-million-square-kilometer deepwater area, according to company Chairman Wang Yilin.